05 May 2010

Back to basics: The "Big Four"

ResearchBlogging.orgThe nice thing about a field season away from all regular internet access is that it gives you a real sabbatical of a sort—a chance to reassess plans and set new goals. One of the new goals I set myself this last field season was to introduce a new kind of topic here at Denim and Tweed.

Most of my writing about science at D&T focuses on recently published discoveries in evolution and ecology. It's fun writing, and it coincides neatly with my regular journal reading, and I intend to keep doing it. But I've discovered that when I want to put new work in context, I often need to discuss fundamental concepts of evolutionary biology that aren't necessarily common knowledge, such as genetic drift or sexual selection. However, I rarely have room to explain these concepts in depth within a blog post devoted to something else.

So maybe the solution is to devote some posts to explaining these "basics." I'm going to start with a series of posts on the "Big Four" processes of population genetics. These are the four processes that account, in one way or another, for every change in the frequency of genes within natural populations. In other words, the Big Four account for much of evolution itself. They are:
  • Natural selection, changes in gene frequencies due to fitness advantages, or disadvantages, associated with different genes.
  • Mutation, the source of new forms of genes;
  • Genetic drift, or changes in gene frequencies that arise from the way probability works in finite populations; and
  • Migration, or changes in gene frequencies due to the movement of organisms from site to site.
Lay readers may be surprised both by what we know, and what we don't, about how these four processes operate in nature. Natural selection is relatively easy to measure, and apparently ubiquitous [PDF] in natural populations—but we don't know how often the resulting short-term changes impact evolution over millions of years. Mutation, the source of variation on which natural selection acts, seems to vary widely among living things. Genetic drift means that a trait can come to dominate a population even if it has no fitness effect—or sometimes a deleterious one. Finally, migration across variable landscapes can interact with selection, drift, and mutation [$a] to completely alter their effects.

I'll devote one post each to selection, mutation, drift, and migration, discussing classic findings as well as more recent scientific discoveries about each. They'll arrive as my usual mid-week science posts for the next four weeks, and I'll update this post with links to the others as they go online—so if this looks worth following, you can either bookmark this post, or subscribe to D&T's RSS Feed.

Natural selection, mutation, genetic drift, and migration act together to shape the evolution of natural populations. Photo by jby.


Drake JW, Charlesworth B, Charlesworth D, & Crow JF (1998). Rates of spontaneous mutation. Genetics, 148 (4), 1667-86 PMID: 9560386

Kingsolver, J., Hoekstra, H., Hoekstra, J., Berrigan, D., Vignieri, S., Hill, C., Hoang, A., Gibert, P., & Beerli, P. (2001). The strength of phenotypic selection in natural populations. The American Naturalist, 157 (3), 245-61 DOI: 10.1086/319193

Slatkin, M. (1987). Gene flow and the geographic structure of natural populations. Science, 236 (4803), 787-92 DOI: 10.1126/science.3576198

Wright S (1931). Evolution in Mendelian populations. Genetics, 16 (2), 97-159 PMID: 17246615


  1. Please send me the links so I can add them to my "Basics" page...

  2. Why, thanks, John—I'll make sure to do that.

  3. I have a question. I have read pars of Michael Lynch's Origins of Genome Architecture. While I had always thought the "four fources" of evolution were mutation, selection, drift and migration as you describe, Lynch seems to replace migration with recombination. Is this valid? Is recombination normally placed under mutation?

    I have very little understanding of population genetics (which I want to fix but alas my college offers no such course!) so I have no idea what to think.

  4. Phylogenous, I have to admit I haven't read the book in question, but I could probably come up with a good case for including recombination on the same footing as the other "four forces." But I don't think I'd drop migration to do it—spatial structure is important in natural populations. (Maybe, if the others go well, I'll write a bonus fifth piece on recombination.)

  5. Well, it is possible that because he is discussing genome architecture, migration may just not be as important (or even undetectable?).